With the development of radio communications, users impose higher and higher requirements on Quality of Service (QoS) and diversification, thus giving rise to numerous Value Added Services (VASs). Being convenient and quickly available, the VASs meet the diversification requirements of the users.
Currently, the technology of multimedia session call transfer enables transfer of a user session and transfer of media streams between User Equipments (UEs), as described below with the Explicit Call Transfer (ECT) service as an example.
The ECT service falls into two types: interrogating ECT and blind ECT:
The detailed process of the interrogating ECT includes the following steps:
A first UE (UE-1) performs a multimedia session with a second UE (UE-2);
UE-1 holds the call with UE-2;
UE-1 establishes a call with a third UE (UE-3), and keeps a conversion with UE-3 for a period;
UE-1 quits the session; and
UE-2 connects to UE-3 for a conversation.
The detailed process of the blind ECT includes these steps: UE-1 performs a multimedia session with UE-2; UE-1 sends the address of UE-3 to UE-2; UE-2 establishes a conversion with UE-3, and UE-1 quits.
With the development of the multimedia transmission technology, the user services are diversified, including not only the voice service, but also the video service and file transfer. The Packet Switched (PS)-based IP Multimedia Subsystem (IMS) will be the development trend.
The current multimedia session technology enables one user to operate multiple UEs simultaneously and perform a multimedia session with the peer. Multiple UEs operated by the user may be integrated, for example, into a dual-mode UE. A user may use multiple UEs to talk with the peer through call transfer performed in the following way:
As shown in FIG. 1, suppose that UE-1, UE-2, and UE-3 are performing a multimedia session. The session includes three types of media streams: voice, video, and text. Two types of media streams are exchanged between UE-1 and UE-3, and one type of media streams is exchanged between UE-2 and UE-3. In this case, UE-3 transfers the session to UE-4.
In FIG. 1, both the Serving Call Session Control Function (S-CSCF) and the Multimedia Session Continuity (MMSC) AS are in the home network of UE-1. The S-CSCF and the MMSC AS in the home networks of UE-2, UE-3, and UE-4 are omitted.
Step 1: UE-3 initiates an ECT request. The ECT request is a Refer, namely, call transfer, message and carries the address of UE-4. This message is transmitted through the S-CSCF of UE-1 to the MMSC AS of UE-1.
The MMSC technology enables a multimedia session to be continuous between different access networks or different UEs. The core of the MMSC technology is the MMSC AS. All calls and sessions of a user need to pass through the MMSC AS of the IMS domain. The MMSC AS acts as a Back-to-Back User Agent (B2BUA) to prepare for the subsequent multimedia session continuity. This process is known as “anchoring in IMS”. All calls initiated by an MMSC user from different access networks or different UEs need to be routed to the MMSC AS to undergo the anchoring process, and then sent to the called party. Likewise, all calls or sessions directed to the MMSC user need to be routed to the MMSC AS for anchoring.
Step 2: The MMSC AS sends the Refer message to UE-2.
Step 3: The MMSC AS sends the Refer message to UE-1.
The MMSC AS is a B2BUA. According to the anchoring information existent at the time of establishing the call, the MMSC AS splits the Refer message into two Refer messages, which are sent to UE-2 and UE-1 respectively through step 2 and step 3.
The B2BUA is defined as two user agents that are connected back to back. For example, in the anchoring implemented by the AS, UE-1 sends an Invite, namely, a call request, message to UE-2 on the peer side. After the call arrives at the AS, the AS terminates the Invite message, generates a new Invite message and sends it to UE-2 on the peer side. In this case, the AS acts as a B2BUA. The connection between UE-1 and the AS is dialog-1, and the connection between the AS and UE-2 is dialog-2. The AS binds dialog-1 and dialog-2 together. The dialog processing mechanism maintains the sequence and the transmission path of the messages between the two UAs. A dialog is a peer-to-peer relation between two UAs.
Steps 4-5:1TE-1 and UE-2 return a 202 OK message respectively.
Step 6: According to the anchoring information, the MMSC AS integrates the two 202 OK messages and sends them to UE-3.
Step 7: After receiving the OK message, UE-3 sends a BYE message to release the session.
Steps 8-9: According to the anchoring information, the MMSC AS sends the BYE message to UE-1 and UE-2 respectively. After step 9, the signaling and media connections between UE-1, UE-2, and UE-3 are cut off.
Steps 10-11: UE-1 and UE-2 send an Invite message to UE-4 respectively.
Step 12: The MMSC AS anchors and correlates the two Invite messages, and sends the Invite message to UE-4.
In the process of developing the present invention, the inventor finds at least the following defects in the prior art:
In the call transfer method in the prior art, a mechanism of disconnecting and reconstructing a session is applied. The existing multimedia session between the party who uses multiple UEs (multi-UE party) and the peer is released, and a new multimedia session is established between the multi-UE party and the third party. The whole process is troublesome and the signaling flow is complicated, and the implementation is time-consuming and inefficient.